This invention relates to a novel class of compounds which exhibit analgesic activity.
Recently, endogenous substances having morphine-like properties have been extracted from mammalian brain or csf. These substances, named enkephalin, have been identified by Huges et al., Nature, 258, 577 (1975) as pentapeptides having the following sequences: EQU H-Tyr-Gly-Gly-Phe-Met-OH EQU H-Tyr-Gly-Gly-Phe-Leu-OH.
These compounds are referred to as methionine-enkephalin and leucine-enkephalin, respectively.
Although methionine and leucine enkephalin have been shown to exhibit analgesic activity in mice upon administration intracerebroventricularly [Buscher et al., Nature, 261, 423 (1976)], they are practically devoid of any useful analgesic activity when administered parenterally.
Therefore, since the discovery of the enkephalins, much effort has been devoted to preparing analogs of the enkephalins in the hope of finding compounds having enhanced activity and practical utility due to their bioavailability by parenteral or oral administration.
Dutta et al., Life Sciences 21, pp. 559-562 (1977) report certain structure modifications which, they suggest, tend to enhance potency. They suggest activity can be enhanced by any or all of the following:
(a) substitution of Gly in position 2 by certain D- or .alpha.-aza-amino acids;
(b) conversion of the terminal carboxyl to the methyl ester or the amide;
(c) modification of the Phe in the 4-position by .alpha.-aza substitution, N-methylation, or hydrogenation of the aromatic ring.
In addition, Roemer et al., Nature 268, pp. 547-549 (1977), suggest modification of the Met.sup.5 to its corresponding carbinol and oxidation of the Met sulfur to the sulfoxide as useful modifications.
Another structural modification of significance is that reported in Belgian Patent No. 859,026. This publication suggests enhancement of activity and bioavailability of enkephalin analogs by insertion of a D-amino acid residue in position 2, conversion of the terminal carboxyl to an amide, and N-alkylation of the amino acid residue in position 5.
A class of analogs of enkephalin having a high level of analgesic activity has now been discovered. These analogs are pentapeptides having the residue of a phenylglycyl or a ring-substituted phenylglycyl in their 5-position.
The literature recognizes other pentapeptide enkephalin analogs having an aromatic amino acid residue in the 5-position. For example, Ling et al., "Structure-Activity Relationships of Enkephalin and Endorphin Analogs", Peptides: Proceedings of the Fifth American Peptide Symposium, John Wiley and Sons, New York (1977), pp. 96-99, discloses Tyr-D-Ala-Gly-Phe-D-Phe-OH. This compound, however, exhibits limited analgesic potential whereas those of this invention, in which the group in the 5-position is a phenylglycyl moiety, have a startlingly high degree of analgesic activity.